Structure, Stability and Vibrational Properties of CdSe Wurtzite Molecules and Nanocrystals: A DFT Study

Abstract

Bare and hydrogen passivated CdSe wurtzite molecules and nanostructures are investigated. The investigation is performed using wurtzoid structures that represent the wurtzite structure at the molecular and nanoscale region. The results show that the energy gap of bare and hydrogen passivated CdSe molecules is higher than and converges to the experimental bulk energy gap. Vibrational analysis of wurtzoid molecules shows that the experimental longitudinal optical mode is in between bare and hydrogen passivated CdSe molecules and very near to bare molecules. The stability of wurtzoid molecules against transition to CdSe diamondoids and cuboids that represent the molecular scale of diamond and rock-salt structures respectively is investigated. The results show that CdSe wurtzoids are the most stable followed by cuboids and diamondoids respectively according to their calculated Gibbs free energy. This order is different than the bulk order of wurtzite, zincblende, and rock-salt respectively. The stability of wurtzoids is attributed to their compact and symmetrical surface structure. The analysis also shows that dangling bonds on bare wurtzoids are responsible for the differences between bare and passivated molecules including lower energy gap, higher vibrational force constants, and lower vibrational reduced masses.